The present invention relates to a speech coding and more particularly to the time-separated speech coder that codes by separating the transitional analyzer after detecting the transitional point of the transitional analyzer in order to obtain more improved speech quality of the transitional analyzer which is not represented well as harmonic speech coding model out of low rate speech coding methods.
Generally there is transitional analyzer in which unvoiced sound is connected to voiced sound or vice versa. As this transitional analyzer has more information about time domain such as abrupt energy variation and pitch period""s variation, in the case of coding method by the harmonic model, there are disadvantages including difficulty of effective coding and occurrence of mechanical synthesis sound.
Concretely there is the transitional analyzer in which voiced and unvoiced sound are together and the transitional analyzer is in the time at which generally voiced sound drift to unvoiced sound or vice versa.
By using linear interpolation overlap/add synthesis method of the harmonic coder in this section, there are disadvantages like the distortion of the pitch and the gain of waveform in the portion in which energy varies not continuously but abruptly. Therefore, the method is required in the transitional analyzer that codes separately after detecting the time at which energy varies abruptly.
Recently the research about coding method of said transitional analyzer has been more important research field according as increase of researches of low rate coding methods. As there is not effective representation technology for the transitional analyzer of the low rate model until now, more appropriate model and coding method are required.
Recently, the research about coding method of said transitional analyzer can be divided into the analysis method in frequency domain and that in time domain.
First, in the analysis method in frequency domain, there is a method for representing the mixed signal of voiced/unvoiced sound using the probability value after obtaining the probability value of the voiced sound by analyzing the spectral of the speech. The U.S. Pat. No. 5,890,108 of Yeldener and Suat, titled xe2x80x9cLow Bit Rate Speech Coding System And Method Using Voicing Probability Determinationxe2x80x9d, describes the contents that synthesizes the mixed signal after analyzing the modified linear predictive parameter of the unvoiced sound and the spectral of the voiced sound according to the degree of the probability value of the voiced sound which is computed by the parameter and pitch extracted from the spectrum of the inputted speech signal. However, this method has a disadvantage of not capable of representing the time information like the time local pulse.
Next, there are methods using sinusoidal wave congregation set which expands the existing sinusoidal wave modeling, for example, the publication issued by Chunyan Li and Vladimir Cuperman in ICASSP 98 volume 2 581-584 pages on May. 1998, entitled xe2x80x9cEnhanced Harmonic Coding Of Speech With Frequency Domain Transition Modelingxe2x80x9d used duplicate harmonic model using several pulse positions, magnitude and phase parameter in order to represent irregular pulse of the transitional analyzer and described the technology for computing each parameter by close loop optimized method. The coding method according to the analysis method in time domain makes total computation to be complicated by applying the harmonic model for several pulse train and by duplicating it and makes effective coding to be difficult without damaging real speech signal.
According to a first aspect of the present invention, a time-separated speech coder for coding the transitional signal of voiced/unvoiced sound through harmonic speech coding is provided. The time-separated speech coder includes an excitation signal""s transitional analyzer that includes a transitional point detector for detecting a transitional point to notify the transitional analyzer of the transitional signal, a harmonic excitation signal analyzer for extracting the harmonic model parameter of the detected transitional analyzer and a harmonic excitation signal synthesizer for adding a harmonic model parameter.
Preferably, the harmonic excitation signal analyzer includes a window for extracting the harmonic model parameter of each block by applying the Time Warp Hamming (TWH) window corresponding to a central point of each block after dividing the Linear Prediction Coefficient (LPC) residual signal, which is one of the inputted signals within the transitional analyzer centering the detected transitional point.
According to a second aspect of the present invention, a time-separated speech coding method for coding the transitional signal of voiced/unvoiced sound through harmonic speech coding includes detecting the transitional point of the transitional signals, extracting a harmonic model parameter from each block by applying the TWH window to the central point of left/right block after dividing an LPC residue signal out of inputted signals centering the transitional point, and adding the harmonic model parameter.